There is a trend today in the automobile industry to operate the various accessories, such as the power steering pump, oil and air pumps, air conditioning and alternator, by a single endless belt driven by a pulley connected to the engine crank-shaft. This system is referred to as a "serpentine" drive belt system. To ensure optimum operating efficiency for these various accessories, it is necessary that the drive belt be maintained at a predetermined tension to assure efficient performance of the accessories as well as satisfactory service life for the belt. Due to the relatively greater length for the single drive belt which replaces the heretofore plurality of smaller belts, there is a greater tendency for the belt to stretch which will affect the operating characteristics of the driven accessories. Therefore, it is desirable that a belt tensioning device be used for these endless belts to provide reliable service over an extended period of time and to maintain a constant amount of tension thereon regardless of the amount of belt stretch.
Numerous devices have been proposed and used to accomplish this purpose. One type of tensioner uses a bushing formed of an elastomeric material which is placed in compression by some mechanical means for continuously exerting a tensioning force on the belt. Examples of these constructions are shown in U.S. Pat. Nos. 3,975,965 and 4,144,772. These tensioning constructions, which use an elastomeric material, have the disadvantages in that the high rate which they exert on the belt results in the rapid loss of tensioning as the belt stretches, and this load rate limits the stroke of the belt engaged idler pulley to a shorter distance than desired. Also, sudden acceleration and deceleration of the drive belt can cause a whipping action to occur which causes excessively high or low tensioning values.
Numerous other types of belt tensioning devices use coil springs which are either in compression or tension, for applying and maintaining the tensioning force on a belt-engaging idler pulley or chain-engaging idler pulley or chain-engaging sprocket. Some examples of these types of constructions are shown in U.S. Pat. Nos. 2,703,019, 2,893,255, 3,413,866, 3,483,763, 3,631,734, 3,768,324, 3,812,733, 3,924,483, 3,965,768 and 4,108,013. Some of these various coil spring-actuated devices use the biasing force of a spring in combination with hydraulic-actuated members for regulating the amount of tensioning force applied to the belt, depending on whether the engine is running or shut off. Examples of these combination spring and hydraulic belt tensioners are shown in U.S. Pat. Nos. 2,051,488, 3,142,193 and 4,077,272.
Other types of tensioning devices and arrangements are provided with some type of mechanical retaining means, usually a ratchet-pawl retaining mechanism, which limits the movement of the belt tensioning member in an opposite nontensioning direction, thereby maintaining a constant tensioning force on the endless drive belt and eliminating the undesirable effects of belt whipping. Examples of these prior constructions and arrangements having such retaining mechanisms are shown in U.S. Pat. Nos. 2,051,488, 2,703,019, 3,413,866, 3,631,734 and 3,812,733.
Other known belt tensioner constructions, such as shown in U.S. Pat. No. 3,924,483, use a torsional spring for pivotally moving one of the vehicle accessories to achieve the desired tensioning force. Other constructions, such as shown in U.S. Pat. Nos. 3,136,170, 3,483,763, 3,834,246, and 4,285,676, use a torsional coil spring for pivotally moving a lever and idler pulley into belt tensioning engagement which provides a relatively simple, economical and compact unit.
It is desirable that a belt tensioner be provided with some type of damping means to prevent excessive oscillation from occurring in the spring tensioning member, and which will absorb sudden shocks to prevent a whipping action from occurring in the tensioner and drive belt. This damping action is especially critical when a coil spring is used for applying the belt tensioning force since coil springs are highly susceptible to developing natural oscillating frequencies when the counter force, which is exerted thereon by the belt, fluctuates during acceleration action affects the efficiency of the tensioning force applied to the belt by the coil spring and reduces belt life. Furthermore, without sufficient damping the tensioner may vibrate which will shorten considerably the life of the drive belt. Also, energy which is absorbed by the belt is imparted to the tensioner and will cause this undesirable vibration and spring harmonies unless damping is provided. Also, damping will prevent noise from occuring in the tensioner caused by the spring harmonies and tensioner vibrations.
Various damping devices have been used with belt tensioners to eliminate or reduce this problem of coil spring oscillation. One type of construction uses a hydraulic fluid as the damping means, such as shown in U.S. Pat. Nos. 2,893,255, 3,964,311, 3,986,407 and British Pat. No. 336,737. U.S. Pat. No. 3,710,634 shows a belt tensioner which uses an eccentrically mounted mechanical pinion and rack arrangement which is spring biased by a leaf spring for absorbing an excessive amount of shock as opposed to providing a damping action for spring-biased belt tensioning plunger.
It also is highly desirable when developing a belt tensioner intended primarily for use on an automobile to devise a construction which can be produced as inexpensively as possible without sacrificing durability and efficiency since a savings of only a part of a dollar would amount to a sufficient overall savings when considering the millions of vehicles that are produced by the various vehicle manufacturers.
Another problem that occasionally occurs with belt tensioners is bearing failure such as the bearing ring which rotatably mounts the idler pulley on the pivotally mounted lever as well as the bearing which pivotally mounts the lever on its shaft which is mounted on the engine or engine bracket. The lever of many belt tensioners is pivotally mounted by a bushing sleeve formed of bronze or similar friction reducing material. Due to the unbalanced force components that are exerted of the idler pulley with respect to the bushing sleeve, the sleeve bushing has to be relatively long in axial length in order to distribute the unbalanced forces along the length of the sleeve. It is desirable that the length of this bushing sleeve be as short as possible to reduce the overall size of the tensioner as well as providing versatility for mounting it on various engine configurations in addition to increasing bearing life. Therefore, the need has existed for such a belt tensioner having an improved mounting arrangement of the idler pulley with respect to the tensioning lever pivot shaft which increases bearing life, which may include damping means, and which reduces the friction forces exerted on the pivot bushing of the lever means by aligning the belt engaging idler pulley with the bushing center line.